Fix layout semantics

[fsimonis]

This PR fixes the landmark semantics of the HTML layout using the Accessibility insights for web inspectors.

• sidebar and table of content navigations are now correctly wrapped in nav tags and use aria-label to describe which label they are
• the main content does not contain elements which are repeated over the website
• the website footer is outside of the main tag
• the sidebar is outside of the main tag
• the news banner is a <aside role="banner"> and outside the main tag
• the edit section is wrapped in an aside

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[MakisH]

Sounds like a good change that should not harm anything.
I only had a quick look, and I do not have enough expertise in web development to judge the technicalities.